Some pond fish species, such as goldfish, koi and some other carp species are more tolerant of cooler temperatures than others and can survive in cold water temperatures between 35-40° F. During the winter these species will become less active and will spend most of their time resting in the deepest parts of the pond, often near the bottom where the water is slightly warmer. Their breathing rate and metabolism will slow down significantly, and they will rely on their body fat reserves for energy.
Most other pond fish species, including tropical and subtropical species like many cichlids and killifish, are cannot tolerate cold water and must remain in heated water in the winter to survive.
To ensure the health and well-being of pond fish during the colder months, it is essential to maintain proper water quality, provide adequate aeration and supply a proper diet to help support them during this dormant period.